1. Field of the Invention
The present invention relates to a process for producing a porous silicon oxide material having mesopores suitable for use as supports for catalysts and the like, adsorbents for hydrocarbons and the like, fixing supports for enzymes and the like, and synthetic sites for functional substances and the like.
2. Description of the Related Art
Silica gel, activated carbon, and amorphous or crystalline porous materials such as zeolites have been widely used heretofore as supports for catalysts and the like, adsorbents for hydrocarbons and the like, fixing supports for enzymes and the like, and synthetic sites for functional substances and the like.
In particular, because pores of uniform size are distributed over the entire structure, the crystalline porous material inclusive of zeolites exhibit excellent characteristics recognized as "shape selectivity". More specifically, for example, a crystalline porous material undergoes selective catalytic reaction, and provides the function of selective adsorption and separation.
The selective catalytic reaction of a crystalline porous material is described in detail below.
Reactant selectivity occurs in case the crystalline porous material is used as a support or a synthetic site or field, because only the reactant molecules smaller than the pore size of the crystalline porous material can be selectively reacted. Otherwise, product selectivity occurs in case reaction products smaller than the pore diameter are selectively produced. By taking the advantage of these types of selectivity, the desired reactant molecules alone can be reacted, or only the intended reaction products can be obtained.
The function of selective adsorption and separation relates to the adsorption and desorption of a gas, or to the trapping of a gas using the aforementioned crystalline porous material. More specifically, a mixture of gases comprising gas molecules differing in diameter is passed through the crystalline porous material, such that a specified gas alone can be adsorbed, desorbed, or fixed from the gas mixture depending on the pore dimension of the crystalline porous material.
Higher performance can be expected in the function of selective adsorption and desorption of the crystalline porous material by increasing the uniformity in pore size, or by increasing the crystallinity as well as the purity of the crystalline porous material.
However, there are problems on the crystalline porous material described above yet to be solved. For instance, the crystalline porous materials described hereinbefore are not suitable for use as a support in a catalytic reaction involving relatively large molecules of, for example, trimethylbenzene or naphthalene. They cannot be used as the adsorbents for adsorbing and separating such relatively large molecules. This is ascribed to the fact that even maximum pore diameter available on a zeolite or the like that is used conventionally as the crystalline porous materials is too small; more specifically, a maximum pore diameter available in a conventional crystalline porous material is 1.3 nm.
Concerning a reaction involving smaller molecules, on the other hand, the crystalline porous materials are sometimes found to function insufficiently as supports for the catalytic reaction and the like. A catalytic reaction in the purification of an exhaust gas can be mentioned as an example for such an insufficient catalytic reaction because the blow rate of an exhaust gas is very high. In such a case, it takes much time for reactant molecules to reach the active sites because the pore dimension of the porous material is too small, the gaseous reactant passes over the catalyst before the activity of the catalyst is fully exerted thereto.
Recently, in the light of the aforementioned circumstances, mesoporous materials characterized by their large and uniform pores have been proposed.
The mesoporous material is characterized by its three-dimensional framework structure, and it contains fine pores of uniform size ranging from 1.5 to 10 nm in diameter.
The mesoporous material can be produced by using a layered silicate or a non-layered silicate such as water glass, tetraethylorthosilicate, or silica as the starting material, and bringing the starting material into contact with a surfactant.
First, the process for producing mesoporous materials by using a layered silicate as the starting material is described in U.S. patent applications Ser. Nos. 08/192,933, and 08/192,962 (both filed Feb. 7, 1994), as including the steps of: (a) introducing an organic substance having a diameter of 10 .ANG. or above by ion exchange into the interlayer space of crystals of layered silicates, and forming interlayer bridges of SiO.sub.2 by condensation of surface silanols; (b) bringing the intercalated compounds into contact with a salt of a metal other than silicon; and (c) firing the products at a high temperature to remove organic composition and to fix the metal ion into the frame works; the steps being carried out in the order of either (a), (b) and (c); (b), (a) and (c); (a) and (b) at the same time and (c), or (a), (c), (b) and (c). These entire references are incorporated herein by reference.
A similar process for producing mesoporous materials by using a layered silicate (kanemite) as the starting material is described in Bull. Chem. Soc. Jpn., vol. 63, pg.
988 (1990). Alkyltrimethylammonium-kanemite complexes were synthesized by the treatment of kanemite with alkyltrimethylammonium chloride solutions. .sup.29 Si-MAS NMR and X-ray powder diffraction clarified that SiO.sub.2 layers in the complexes were condensed with each other to form three-dimensional SiO.sub.2 networks. The calcined products of the complexes have micropores whose sizes increased with the length of the alkyl chain in the alkyltrimethylammonium ions used. These facts indicated the synthesis of porous SiO.sub.2 with a controlled pore size. This entire reference is incorporated herein by reference.
A generalized description of these methods, including a proposed mechanism follows
A layered silicate such as Kanemite is heated in an aqueous solution containing the surfactant dissolved therein. During this process step, the surfactant is introduced into the interlayers of the layered silicate to form micelles of the surfactant in the interlayers of the silicate. Furthermore, the silicate sheets constituting the layered silicate bend around the surfactant to partially combine with each other. In this manner, the silicate forms a three-dimensional structure to provide a composite of the surfactant and the three-dimensional silicate.
The composite thus obtained is separated from the aqueous solution, and is washed and dried thereafter.
Finally, the composite is subjected to the calcination process to obtain a mesoporous material. More specifically, by calcining, the surfactant is removed from the interior of the composite, and the pores are formed inside the composite.
The mesoporous material thus obtained is composed of silicate sheets which originally constituted the layered silicate used as the staring material, and the silicate sheets are bent and are partially combined with each other to form a three-dimensional structure. The resulting mesoporous material contains fine pores of uniform size ranging from 1.5 to 10 nm in diameter.
Then, the process for producing porous materials by using a non-layered silicate as the starting material is described in U.S. Pat. No. 5,057,296 as follows. The reference is incorporated herein by reference.
A non-layered silicate such as water glass or tetraethylorthosilicate or an amorphous silica is heated in an aqueous solution containing a surfactant dissolved therein. During this process step, the silicate particles aggregate around rod-like (micelle-like) surfactant, and the resulting silicate aggregates polymerize to form a three-dimensional structure by the heat applied thereto. In the same manner as in the production process using the layered silicate above, a composite of the surfactant and the three-dimensional silicate can be obtained.
The composite thus obtained is separated from the aqueous solution, followed by washing and drying thereafter. Finally, the composite is subjected to a process step of removing the surfactant from the composite and forming fine pores therein.
The mesoporous material thus obtained is composed of three-dimensional structure of silicate sheets, and contains fine pores of uniform size ranging from 1.5 to 10 nm in diameter.
In both of the processes described above, the composite is formed by heating a starting material comprising the silicate in an aqueous solution containing a surfactant dissolved therein. However, no operation of adjusting the pH value of the aqueous solution is effected in both of the processes.
The mesoporous materials produced according to the process above still have the problems below yet to be solved.
That is, the mesoporous materials thus obtained are inferior in crystallinity; moreover, the pores thereof are not uniform in size. Accordingly, superior shape selectivity cannot be expected. Furthermore, the mesoporous materials are insufficient in heat resistance.
Thus, they are not applicable to, for instance, cracking catalyst supports for petroleum, or catalyst supports for purifying exhaust gas discharged from automobiles. Even in calcination, the final step of production, the crystal structure and pore of the material may be destroyed by heat.
Furthermore, the mesoporous materials contain impurities at a considerable quantity. This decreases the concentration of mesopores and deteriorates the function of a porous material. Moreover, amorphous silica containing the impurities impairs the crystallinity of the mesoporous material.